I've been experiencing the same problem. Irritating how it happens. I've also been looking around for answers, but seemingly it's also unavailable.
If it cannot attach I often do this:
1. Cancel draft, starting new, attach the file first before starting to compose or add recipients address
2. If it's should be on a thread, close the page, re-open in other browser. I know it sounds stupid, but it works.
I am no IT savvy or whatever, but this is what I do.. :-/